FIG. 1 shows an exemplary network having a packet switched wide area network (WAN) 6 used to transport, for example, enterprise traffic between remote corporate campuses 10, 11. Associated with each remote campus 10, 11 are edge nodes 5, 7. Edge nodes 5, 7 are used as gateways between the WAN 6 and the campus equipment 4, 8.
Note that telephony equipment may be part of the campus 10, 11 network. Specifically telephones (e.g., telephone 1), FAX machines (e.g., FAX machines 3, 9), PBXs (e.g., PBXs 4,8) or other device (such as a modem) used to send receive information over a time division multiplexed (TDM) network, such as the telephone network, are part of the overall network formed by attachment to the packet switched WAN 6.
In cases where telephony equipment is serviced by a packet switched WAN, the edge nodes 5,7 typically perform some form of TDM/packet translation. Because the telephony equipment is designed to interface with a TDM network, the edge nodes 5,7 provide a TDM network interface 12, 13 for this equipment. Thus edge nodes 5, 7 translate the telephony traffic to/from a packet switched network interface 14, 15 from/to a TDM network interface 12, 13.
Edge nodes 5, 7 also perform various forms of compression in order to reduce bandwidth consumption of the WAN 6 by the telephony traffic. Typically the TDM network interface 12, 13 comprises a 64 kb/s channel for each telephony line (e.g., telephony line 2). Telephone lines may also be referred to as channels. The edge nodes 5, 7 are responsible for reducing the per channel bandwidth below 64 kb/s. This results in more efficient utilization of the WAN 6.
For example, an outgoing telephone call that consumes one telephony line is usually compressed from 64 kb/s to 32, 16 or 8 kb/s by a compression algorithm executed by a digital signal processor (DSP) within the edge node (e.g., edge node 5). The outgoing call then enters the WAN 6 as a 32, 16 or 8 kb/s offered load (not counting overhead) resulting in a 32, 48 or 56 kb/s savings in WAN 6 bandwidth. The receiving edge node (e.g., edge node 7) decompresses the phone call back to 64 kb/s and sends the call over TDM interface 13 to PBX 8.
Because FAX (also referred to as facsimile) traffic typically consumes a single telephony line (e.g., telephony line 17), a FAX call also receives up to 64 kb/s of service at the TDM interface 12. In order to reduce WAN 6 bandwidth consumption, the FAX call may be “compressed” or otherwise reduced. A technology known as FAX Relay, mostly embodied in ITU standards T.4, T.6, T.30, V.17, V.21, V.27, V.29 and V.33, specifies the manner in which a 64 kb/s FAX data stream is reduced to a lower bit rate (such as 0.3, 2.4, 4.8, 9.6, and 14.4 kbps). Fax Relay involves identification of a baseband image signal from the 64 kb/s stream. Edge nodes 5, 7 may therefore also perform (besides voice compression/decompression) FAX Relay processing functions for FAX calls in order to reduce WAN 6 bandwidth consumption while maintaining a TDM interface 12, 13 for the telephony equipment.
A problem with implementing Fax Relay is the relative newness of FAX Relay for packet switched networks. Existing packet switched WANs have a sizable installed base of equipment without Fax Relay capability. Thus a transmitting edge node (e.g., edge node 5) and a receiving edge node (e.g., edge node 7) should first communicate with one another to ensure both have the ability to process Fax Relay signals.
A packet network is any network that transports data with packets. This network includes, for example, a FastPacket™ network from Cisco Technology Inc. A packet network may also be a WAN, Local Area Network (LAN) or regional network such as those associated with local exchange carriers. A packet network also may include two or more FastPacket™ networks bridged by an IP, Frame Relay, or ATM network. The bridging network (e.g., IP, Frame Relay, or ATM networks) couples the FastPacket™ networks.
Telephony control signals are distinguishable from telephony data signals. In a sense, telephony control signals are procedural whereas telephony data signals are substantive. Telephony control signals help setup and maintain the connection between telephony equipment (such as PBXs, telephones, modems and FAX machines) while telephony data signals are the substantive traffic (e.g., a telephone conversation between users) carried over the connection. This distinction between substantive and procedure is maintained with reference to packet technology discussed below.
For packet based WANs 6, the “connection” is not an actual connection since TDM technology is not employed. A protocol referred to as the Transition State Protocol (TSP) is a special form of packet based communication used to communicate telephony control signals over a FastPacket™ WAN 6.
For telephony based communications (e.g., telephone calls, FAX transmissions, etc.), control signals that indicate 1) the telephone number is dialed, 2) the telephone number is answered, 3) the receiver is off hook, and 4) the receiver is on hook are communicated over the WAN 6 by way of TSP packets and the TSP protocol. TSP packets are control packets sent over the WAN 6 to communicate these control signals between transmitting and receiving telephony equipment. A more detailed description of TSP technology for these telephony control signals is described in U.S. Pat. No. 5,115,431 entitled “Method and Apparatus For Packet Communications Signaling” issued May 19, 1992.
Referring again to FIG. 1, an originating edge node (e.g., edge node 5) needs to know if the destination edge node (e.g., edge node 7) has the ability to process, for example, a 14.4 kb/s Fax Relay signal from the WAN 6 up to a 64 kb/s traditional FAX signal for delivery to the PBX 8. If so, the originating edge node 5 can transmit the 14.4 kb/s Fax Relay signal over the WAN 6. If not, the edge node 5 may transmit a full 64 kb/s traditional signal over the WAN 6 instead since the bandwidth efficiency offered by FAX Relay is not available for this particular connection.
However, the knowledge process starts with the destination edge node 7 which results from the manner in which FAXes are transmitted. A brief discussion of FAX transmission as compared to other transmission types is helpful.
At least three types of information have traditionally been transmitted over networks: data, voice and FAXes. Voice data may be compressed since telephone users can still comprehend a conversation with less than 64 kb/s of bandwidth between them. Data information is typically transmitted via a data modem (which is distinguishable from a FAX) and may be compressed as well. Similarly, FAXed traffic may be compressed (or otherwise processed by FAX Relay) because the FAX “baseband” signal needed to reproduce the image may be transmitted with less than a 64 kb/s channel. Nevertheless, the compression or reduction methods are different between FAX Relay, data, and voice transmission. Furthermore, for edge node pairs that cannot support FAX Relay, the FAX connection is typically implemented with 32 kb/s or a full 64 kb/s effective offered load to the packet network. Thus, regardless if FAX Relay is implemented or not, a destination node needs to distinguish between a voice call, a FAX call, and a data call.
FAX and data modem connections begin with a special tone (referred to as a Called Station Identification (CED tone)) generated by the receiving device. The receiving FAX machine or data modem “answers” the call initially with a CED tone. Thus, to first order, voice calls are distinguished from FAX and data modem calls by the presence of the CED tone. FAX calls are further distinguished from data modem calls (since both are answered with the CED tone) by a v.21 preamble. Thus a destination edge node can identify a FAX call within one of its channels by first recognizing a CED tone “answered” by the FAX machine and then recognizing a v.21 preamble. FAX Relay processing may be implemented after the tone and preamble are sent. The FAX call's tone and preamble may be transmitted as a regular voice call because the voice compression algorithms do not distort the tone or preamble to the extent that it is unintelligible at the remote FAX machine.